Nitration of aliphatic hydrocarbons



Patented May 20, 1952 NITRATION F ALIPHATIC HYDROCARBONS Gustave B. Bachman, West Lafayette, Ind., and

James V. Hewett, Columbia, S. 0., assignors to Purdue Research Foundation, La Fayette, Ind.,

\ a corporation of Indiana No'Drawing. Application August 18, 1950,

1 7 Serial No. 180,296

The present invention-relates to the nitration of aliphatic hydrocarbons. More particularly, it'pertainsto the production of nitro hydrocarbons by the vapor phase nitration of aliphatic hydrocarbons at elevated temperatures by means of nitric acid, oxides ofnitro'gen, and the like, in the presenceof regulated amounts of a halogen or a halogen-containing compound, as more particularly described hereinafter.

1 Numerous efforts have been made in the past to develop-a satisfactory method of producing nitro hydrocarbons. Jlhe most effective previously disclosed processes" are those of Hass et al. described in U. S. Patent Nos. 1,967,667, 2,071,122, and 2,206,813, pertaining to the vapor phase nitration of parafiin hydrocarbons such as ethane, propane, butane and the like, by means of nitric acid ornitrogen dioxide, and that of Landon, described in U. S. Patent No. 2,161,475, pertaining to the nitration of methane by means of nitric acid.

All of the prior art processes are open to a number of serious objections, the most important of which have been the low yields based on hydrocarbons used and low conversions based on the nitric acid usedin the process. Because of the cost, corrosiveness, and the like of nitric acid, it is highly desirable to use as small a quantity as possible. We have now discovered that the conversion of nitric acid and yields of nitro hydrocarbons, as well as the production of valuable, oxidation products when hydrocarbons are nitrated in the vapor phase, can be materially increased by efiecting the vapor phase nitration of the hydrocarbons in the presence of a halogen or a-halogen-containing compound in regulated amounts under certain restricted conditions of time, temperature, etc.

Another aspect of our invention involves the .use of regulated amounts of molecular oxygen in addition to the halogen or halogen-containing .compound. .Theuse of oxygen in the manufacture of nitroalkanes by the vapor phase nitration of paraffin hydrocarbonds is known, being describedand claimed incopending application, U. S..Serial No. 15,504, filed'March 17, 1948, now

. abandoned, by Hass and Alexander. It has not been previously known, however, ,that greatly improved yields and conversions of nitro hydro- 1 carbons can be obtained by the use of a halogen,

either in the presence or absence of molecular oxygen. During the course of the investigation which led tothe present invention we dis- ;covered that improved yields andconversions of nitro hydrocarbons are obtained when a free 21 Claims. (Cl. 266-644) halogen is present during the nitration reaction, whether 'or not molecular oxygen is also present.

The improved process whereby we obtain increased yields and conversions of both nitro hydrocarbons and oxidation products such as alcohols, aldehydes, acids, etc., consists essentially of conducting the vapor phase nitration of aliphatic hydrocarbons at temperatures ranging from 200 to 500 C. with a nitrating agent such as nitric acid in contact with a free halogen, or halogen-containing material, and in the presence or absence of molecular oxygen. For each mole of nitric acid used, we have found that from about '7 to about 35 moles of hydrocarbons, from about .0018to .026 mole of freehalogen and not more than about 3.0 moles of molecular oxygen should be used. By. carrying out the nitration under such conditions, the conversions to nitro hydrocarbons are increased by as much as 50% over those obtained by the method of nitration inthe presence of oxygen, and by more than over those obtained. by the conventional methods of nitration in the absence of oxygen. Such lincreases permit the production of nitro hydrocarbons at materially lower cost because of plicable with advantage to other aliphatic hydrocarbons, saturatedand unsaturated, such for instance as the cycloparaffins, and olefins and acetylenes having 3 or more carbon atoms in the molecule. 1

Our process may be satisfactorily carried out over a wide temperature range, i. e. 200 to 500 C., depending upon the hydrocarbon being nitrated, the products desired and other conditions. However, the reactiontime must be altered in accordance with the temperature used; substantially longer. times-being requiredfor the lower temperatures, and substantially shorter times being required for the higher temperatures for optimum conversion of nitric acid to nitro hydrocarbons. The particular hydrocarbon beent. Accordingly. in carrying out our improved process the temperatures, reaction, times, and space velocities are regulated in the same general manner as in the prior art processes.

In general, we have found that theoptimum sions resulting, but the amount of the increase may not be great enough to warrant the use of more halogen. Smaller quantities than .0018 mole of halogen can also be used but diminishing effects on the yield of nitro paraflins may be encountered.

.We have found that any halogen can be used in our process although the use of fluorine is not practical due to its highly corrosive nature. Excellent results are obtainable when chlorine, bromine or iodine is employed as the catalyst. The catalytic action of the halogens is produced only by the free, elements, and it is therefor desirable. to add the halogens as such; that is, un-

contact times vary inversely with the temperatures employed. As the temperature is raised from 200 toward 500 C., the optimum timeof contact decreases from about 15 seconds to slightly less than 1 second. The optimum time of contact, for a given reaction temperature can most conveniently be determined for a given reaction mixture by regulating the time of contact for the reaction mixture at the selected reaction temperature so as to maintain the acidity of the reaction products within a relatively narrow range. Since water is one of the reaction products of the nitration reaction, and the nitro hydrocarbons are. in all cases at least partially immiscible. with water, the liquid reaction products separate into two layers. Thetitratable acidity of the aqueous, layer constitutes a convenient measure of the acidity of the reaction products, and: this; value may be used as a basis for the control'of the time-temperature factor. If the acidity of the aqueous layer of the reaction products rises above the desired operating range, the reaction temperature should be increased, or the contact time increased (space velocityv decreased), or both of these changes may be made simultaneously. Similarly, if the acidity of the aqueous layer drops below the desired operatingv range, the reaction temperature; should be. decreased, or the: contact time should be decreased (space velocity increased), or both. of these changes, may be made simultaneously. Since it is usually desirable to operate. at constant space velocity, it is most convenient merely to adjust the reaction temperature in accordanceawiththe acidity-of the aqueous layer. Very small adjustments.- in temperature will cause relatively large fluctuations of the acidity, and the temperature adjustments should therefore be made gradually. The acidity of the aqueous layer of the product (expressed in normality), should be maintained within the range of 0.1 N. to 1.5 N in order to obtain best results. The'optimum value within this range, in any given case, will. depend upon the particular hydrocarbon being nitrated, and on the composition of the reaction mixture. This general procedure is more. completely disclosed in U. S. Patent No. 2,327,964 by Edward B. Hodge, granted August 24, 1943.

While the exact amount of halogen to be added varies somewhat with the temperature, the hydrocarbon to nitric acid ratio, the amount of oxygen, if any, the contact time, and the type of reactoremployed, it can be said in general that greatly improved yields and conversions are obtained when catalytic proportions of from about .0018 to .026v mole'of a halogen isadded per mole of nitric acid. Larger quantities than .026. mole can bev added with increased convercombined with other elements. Thus chlorine, iodine and bromine can advantageously be added as their gases and iodine as solid element. In-

asmuch as most halogen-containing compounds are readily oxidized by nitric acid under the conditions of the nitration reaction, we have found that we are not limitedto the use of the free ele nts, but, can use any compound which is diz dby nit ic. acid, gi e a free. a og and which doesnot; form other substances which are harmful to the nitration reaction. Bertieularly Suitable, or h s p se ar t e. hyd o e halides, HBr, HQ], and HL the alkyl halides, and acyl halides. Also suitable are the compounds composed of two halogens, and 161-. The amounts of halogensgiven above, fer example .0018 to .026 mole; permole of nitric acid, refer o t e a eelelsm n s so. a t op imum mo e r r; ha n-c ntain n ompoun s ust be calculated fron'rthe number ofhalogen atoms nta ne n; each.molequ eof heparticular com.- p und- I h m thod f mpl in hal e in 1 imp ov oce s may e varied. o what without departing. from the concepts of our invention. For example, the, halogen or halogen containing compound can be added to the hydrocarbon before or after. the introduction. of thev nitric; acid. We have.generallythoughtit advantageous, how'- ever, toyaporize the nitric acid. into. thehydrocarbon and then to addfthe halogen. to this. mixture before heating to. reaction temperature. On other occasions we have found it advantageous to preheat the; halogen-hydrocarbon mixture for several seconds at a temperature of fromv 300: to 500 C. before adding nitricaci'd. Excellent. re sults 'arealso obtained when the halogen or halo.- gen-con'taining compound is added to the nitric acid prior to. entry into'thereaction zone.

The concentration of the nitric acid used may be. varied over a wide range without'materially affecting the results, 1.9%. nitric acid giving re.- sults equally as good as70%. nitric acid. We have found that excellent results: are obtained with nitric acid-i having a. concentration within the range 19 to.85.%.

Quite unexepectdly: we have found that molecular oxygen; when. used together with. halogens, gives increased yields and conversions when used in amounts only up; to 3.0: moles per mole of nitrio acid; When larger quantities. of oxygen. are added, thecatalyticlaction of." the halogens. is completely defeated-and yields and conversions drop sharply. This limit on the amount of ox-ygen which. can. be used is. entirely. unexpected and isdirectly contrary 'to, thesituation with. oxygen when no halogen. is present. For example, U; S. Serial No. 15.505; now Patent No.'2',573,662, discloses that nov upper limit to.theincrease of conversion has beenfoundjasthe: proportion of oxy. gen is.raisedztoward.the exploslvelimit.

The molecular oxygen used in our process may be added either as s'ubstantially pure oxygen or as a mixture with other gases. Another surprising aspect of the present-invention, however, lies in the discovery of the fact that when air is used as the oxygen source the catalytic efiect of the halogens is sharply reduced. This is not to be construed as meaning that air cannot be used as the oxygen source since some catalytic action is eifected even in the presence of air; that is, the yields and conversions are substantially greater than when no halogen is present. This discovcry, that air is not as effective as substantially pure oxygen, is in direct conflict with the teaching of U. S. Serial No. 15,504, wherein it is disclosed that equally good results are obtained with 3 air as with substantially pure Ox gen.

The process of incorporating oxy en in the reaction mixture can be varied in accordancewith different procedures outlined above for incorporating halogens. The particular method used for adding the oxygen is not a critical feature of our invention, and any of the methods known to the art can be used advantageously.

Although our process has been described generally with reference to nitric acid as the nitrating agent, nitrogen dioxide can also be used and halogens exert the same beneficial efiect when employed with this nitrating agent. Slightly differen't conditions are required, however, for using nitrogen dioxide in our process.

As previously indicated, our process gives rise not only to increased conversions and yields of nitro hydrocarbons but also to the increased formation of oxidation products from the hydrocarbon being used. Increased amounts of these oxidation products represent one of the valuable advantages of our process in view of the fact that it gives rise to the formation of sufiicient products of this character to justify their recovery from the It is especially noteworthythat by far the largest increases in yields of oxidation products is that for aldehydes. Therefore, it can be said that the gain in yields of nitro hydrocarbons has been made at the expense'of substantially worthless by-products.

A better understanding of the nature and concepts of our invention can be had by reference to the following examples, which are presented for purpose of illustration only and are in no way intended to limit the invention:

EXADJPLE I A reaction mixture consisting of 20.4 moles of propane and 1 mole of 19% nitric acid was passed in the vapor phase through a reactor consisting of a 40-foot coil of Pyrex. glass tubing (7 mm. diameter) immersed in av fused salt bath at a temperature of 423 C., the contact time being about 1.8 seconds.

In a similar run there was passed through the identical apparatus a reaction mixture consisting of 22.8 moles of propane and 1 mole of 19% nitric acid and 0.015 mole of bromine at a temperature of 423 C. and a contact time of 1.9 seconds. h

The results of these two runs, given in Table I below, show that, without any oxygen being added, the addition of 0.015 mole of bromine to a nitration reaction gives an increase in yield based on nitric acid and conversionbased on Pr p ne.. i

water-soluble fraction of the nitration reaction.

.TABLE I The effect of bromme on the nitration reactzon without oxygen Run 14 15 Temperature, C 423 423 Contact time, sec 1. 8 1. 9 Mole Ratios CsHa/HNOz. 20. 4 22.8 Mole Ratios BH/HNO: 0 0. 015 Percent ConversionHNO;to BNO, 23. 0 27. 0 Yields on Propane (percent):

RNOa 27. 4 50. 0 00 2. 7 none Propylene 24. 3 4. 5 Ethylene 23. 4 15. 0 CO 12.6 19. 5 Carbonyl Compounds 9. 5 11. 0 Moles Recovered:

q 0. 0425 0. 0459 C01 0. 0119 none Propylene .1 0.0358 0.0039 Ethylene I 0.0517 0.0196 00 0. 0556 0.0510 Carbonyl Compounds 0. 0349 0.0240

EXAMPLE II.

To determine the effect of varying amounts of oxygen on the catalytic action of bromine in the nitration of propane, additional .runs were made in the same reactor described in Example I. In these runs the amount of bromine added was maintained constant. The results. shown in Table II, indicate that the addition of oxygen is beneficial only so long as the mole ratio of oxygen to nitric acid is maintained below about 3.0. The optimum mole ratio of oxygen to nitric acid is proven to be about 1.0 for a bromine to nitric acid mole ratio of 0.015.

TABLE II The efiect of varying org gen feed in nitmtions with bromine Run 15 18 17 16 Temperature, "C 423 423 423 423 Contact Time, Sec 1. 9 1. 7 1. 5 1. 8 Mole Ratios:

caHg/ou 11. 5 8. 2 5. 1 C3Ha/HNO3 22. 8 10. 6 9. 9 16. 5 N 0 0. 92 1. 2 3. 1 15.0 15. 0 l5. 0 0. 015 0. 015 0. 015

57. 0 55. 5 33. 2 0. 2 none 0. 5 7. 3 9. 7 16. l 7. 8 4. 1 7. 6 1. 0 3. 6 13. 3 Carbonyl Compoun 11.0 26. 6 27. 0 29. 4 Moles Recovered 1 RN Oz 0. 0459 0. 1371 0. 1765 0. 1030 none 0. 0017 none 0. 0042 0. 0039 0. 0164 0. 0294 0. 0473 0. 0196 0. 0262 0. 0184 0. 0334 0. 0510 0. 0065 0. 0331 0. 1170 Carbonyl Compounds 0. 0240 0 1490 0. 2445 0. 2170 1 All adjusted to same run time.

EXAIVIPLE III Another series of runs was made in the reactor described in Example I, but with a difierent bromine to nitric acid ratio, and at varying temperatures. The results in Table III, serve to illustrate how each of the many variables afiect the yields and conversions when 'using bromine. This example shows that for the particular conditions employed. a temperature of 418 Q. is the optimum for nitration.

Taste III Run 20 21 22 Temperature, 425 418 114 Contact Time, Sec 1.6 1.7' 1.7 Mole Ratios:

CaHs/Oz 9. 8 12. 9 9 O1H1/HNO1, 11. 7 13.7 13. 7 Oz/HNOa 1 2 '1. 1 I. 1 ,H20/HNO3. 1,4 v '1. 4 1.;4 Ian H1003 0. 0028 0.002s 0.002s Per cent Conversion-EN 52:7 53. 2 48. l Yields on Propane (per cent):

RNO '43. 1 59. 4 01. 0 3:0 10.15 2.2 13.0 7.7 20.2 8.5 0.1- 0.0 9.7. 2.0 0.6 22.2 24.3 15.9

EXA' MPLE IV A reaction mixture consisting of 7.14 moles of propane and 1 mole of nitric acid was passed through a reactor as described in Example I. In this run chlorine was added to the reactor as a catalyst. The chlorine was added to nitric acid as a water solution containing approximately 0.66 g. of chlorine to 100 g. of water, thus diluting 7 1. 1% nitric acid to 19% nitric acid. The results and conditions of this run are given in Table IV;

A series of runs was made in the same reactor described in Example I, with chlorine added as catalyst. In these runs all conditions were maintained approximately constant, except that the temperature was varied a few degrees between runs, the mole ratio of oxygen to nitric acid was varied and the propane to nitric acid ratio was varied. For comparative purposes run No. 10 was made with no halogen added and with a minimum amount of oxygen. The results and conditions are given in Table V.

TABLE V Run 4 5 0 7 1 10 Temperature, C 420 423 419 415 419 Time of Rum-min- 30 30 30 Mole Ratios:

1s. 1 10. 1 13.8 4. 73 1.42 1. 42 1.42 1.50 Lax 1.83 1.58 0.67 0.0102 0.0102 0.0102 none 1. 1.69 1.70 .1. 7-1

.1 v 44.7 50.0 50.3 34.3 .3 21.7 21.0 20.1 20.1 .0 2.2 .9 1.3 1.7 .9 2.0 3.7' 7.1 2.8 .1 16.9 21.9 33.0. 20.1 Ethylene 7 .1 I 6.1 1.3 10.2 11.9

Nltrating Agent 5 h 1 p HNO:(percent).' 71.1. 71.1 71.1 .71.; 09.9

sang as 7 EXAMPLE VI 7 A reaction mixture consisting of 12.9 moles of propane and 1 mole of nitric acid was passed through a reactor as described in Example I. In this run iodine was introduced into the reactor as a catalyst. The iodine was introduced as a mixture with the propane, the propane being passed through a tared containing crystalline iodine. The amount of iodine vaporized was controlled by varying the amount of iodine in the bulb and the iodine-propane contact surface. The results and conditions of this run are given in Table VI.

EXAMPLE VII The use of alkyl halides as a source of halogen is demonstrated by the nitration reported in Table VII below. In this run the nitration equipment described in Example I was used and the same procedure employed except that bromine was introduced as ethyl bromide. The ethyl bromide was dissolved in the concentrated nitric acig and introduced into the reaction 'with the am TABLE VII Temperature C 424 'Time of run1nin. 35 Mole ratios:

Q3I-Is/HNO3 13.2

H2O/HNO3 1.55

O2/HNO2 1.32

Ethyl Bromide/I-INOa 0.0128 Contact time-sec 1.77 Per cent conversion-HNO: to RN'Oz 51.3 Yields on propane (per cent) RNOz 50.4

Aldehyde 32.3

Propylene 9.2 Ethylene 1.5 Nitrating agentp'er cent HNO; 69.3

It is to be understood, of course, that we are not limited to the specific operation procedure set forth in the above specific examples since various modifications of the procedure set forth therein will naturally occur to those skilled in the art. For example, the process may be operated in a cyclic manner if desired. Various methods of bringing the reacting gases to the desired reaction temperature may be employed. More concentrated solutions of nitric acid than those shown in the specific examples may also be used. Still other variations common to the prior art may be employed and it is distinctly understood that we may so vary our claimed invention so long as the particular improvement hereinbefore disclosed and covered by the appended claims is followed.

We claim:

1. In a process for the production of improved yields of nitro hydrocarbons by the vapor phase nitration of aliphatic hydrocarbons, the improvement which comprises contacting about 1 mole of nitric acid with about 7 to 35 moles of an aliphatic hydrocarbon and from .0018 to .026 mole of a halogen selected from a group consisting of chlorine, bromine and iodine, said mixture being maintained at a temperature between about 200 and 500 C. for a suflicient period of time to permit substantial nitration of the hydrocarbons.

2. In a process for the production of improved yields of nitro hydrocarbons by the vapor phase nitration of aliphatic hydrocarbons, the improvement which comprises contacting about 1 mole of nitric acid with about 7 to 35 moles of an aliphatic hydrocarbon, not more than 3.0 moles of oxygen and from .0018 to .026 mole of a halogen selected from the group consisting of chlorine, bromine and iodine, said mixture being maintained at a temperature between about 200 and 500 0., for a sufl'icient period of time to permit substantial nitration of the hydrocarbons.

3. The process of claim 2 wherein the reaction time is from 1 to 15 seconds.

4. The process of claim 2 wherein the reaction time is regulated such that the acidity of the aqueous reaction products is maintained at a substantially constant value which is between 0.1 N and 1.5 N.

5. The process of claim 2 wherein the hydrocarbon is a saturated aliphatic hydrocarbon.

6. The process of claim 2 wherein the hydrocarbon is a saturated aliphatic hydrocarbon having a boiling point below about 200 C.

7. The process of claim 2 wherein the hydrocarbon is an acetylene having 3 or more carbon atoms.

8. The process of claim 2 wherein the hydrocarbon is an olefin having 3 or more carbon atoms.

9. In a process for the production of improved yields of nitro hydrocarbons by the vapor phase nitration of aliphatic hydrocarbons, the improvement which comprises contacting in the gaseous phase, about 1 mole of nitric acid with about 7 to 35 moles of an aliphatic hydrocarbon, less than about 3.0 moles of oxygen and from .0018 to .026 mole of a halogen selected from the group consisting of chlorine, bromine and iodine, said gaseous mixture being maintained at a temperature between about 200 and 500 C., for about 1 to 15 seconds, and separating the nitro hydro carbons from the reaction mixture.

10. The process of claim 9 wherein the hydrocarbon is a saturated aliphatic hydrocarbon.

11. The process of claim 9 wherein the hydrocarbon is propane.

12. The process of claim 9 wherein the hydro carbon is butane.

13. The process of claim 9 wherein the hydrocarbon is ethane.

14. The process of claim 9 wherein the halogen is chlorine.

15. The process of claim 9 wherein the halogen is bromine. I

16. The process of claim 9 wherein the nitric acid has a concentration of between about 19 and 17. The process of claim 9 wherein the halogen is introduced as a halogen-containing compound.

18. The process of claim 9 wherein the halogen is introduced as a compound selected from the group consisting of halogen hydrides, alkyl halides and acyl halides.

19. In the vapor phase nitration of hydrocarbons with a nitrating agent selected from the group consisting of nitric acid and nitrogen dioxide, the improvement which comprises conducting the nitration reaction with the reactants in contact with catalytic proportions of a com pound selected from the group consisting of the free halogens, chlorine, bromine and iodine, and oxidizable compounds of these halogens.

20. In the vapor phase nitration of hydrocarbons with a nitrating agent selected from the group consisting of nitric acid and nitrogen dioxide, the improvement which comprises conducting the nitration reaction with the reactants in the presence of oxygen in contact with catalytic proportions of a compound selected from the group consisting of the free halogens, chlorine, bromine and iodine, and oxidizable compounds of these halogens.

21. In the vapor phase nitration of hydrocarbons the improvement which comprises mixing at elevated temperatures a nitrating agent selected from the group consisting of nitric acid and nitrogen dioxide, with an aliphatic hydrocarbon and catalytic proportions of a compound selected from the group consisting of the free halogens, chlorine, bromine and iodine, and oxidizable compounds of these halogens, and maintaining said mixture at elevated temperatures for a sufiicient period of time to permit substantial nitration of the hydrocarbons.

/ The following references are of record in the file of this patent:

FOREIGN PATENTS Country Date Great Britain Nov. 30, 1948 Number 

1. IN A PROCESS FOR THE PRODUCTION OF IMPROVED YIELDS OF NITRO HYDROCARBONS BY THE VAPOR PHASE NITRATION OF ALIPHATIC HYDROCARBONS, THE IMPROVEMENT WHICH COMPRISES CONTACTING ABOUT 1 MOLE OF NITRIC ACID WITH ABOUT 7 TO 35 MOLES OF AN ALIPHATIC HYDROCARBON AND FROM .0018 TO .026 MOLE OF A HALOGEN SELECTED FROM A GROUP CONSISTING OF CHLORINE, BROMINE AND IODINE, SAID MIXTURE BEING MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 200 AND 500* C. FOR A SUFFICIENT PERIOD OF TIME TO PERMIT SUNSTANTIAL NITRATION OF THE HYDROCARBONS. 